Diamond Properties : Hardness

Diamond is the hardest naturally occurring substance known; it is also the most popular gemstone. Because of their extreme hardness, diamonds have a number of important industrial applications. Hardness is the measure of a substance's resistance to being scratched, and only a diamond can scratch another diamond.

Hardness is the characteristic of a solid material expressing its resistance to permanent deformation. There are three principal operational definitions of hardness:

• Scratch hardness
• Indentation hardness
• Rebound, dynamic or absolute hardness

The Mohs scale a hardness scale developed in 1822 by Austrian Friedreich Mohs as a criterion for mineral identification can help us appreciate the hardness of diamond. The scale ranks 10 minerals; harder minerals, with a higher number, can scratch those with a lower number.

Scratch hardness (Mohs Scale):

The Mohs hardness is a scratch hardness test and is related to the indentation hardness of the solid. If the Mohs number is M and the indentation hardness in kg/mm2 is H, the relation between these quantities is approximately

There is reasonable 'equality of intervals' between the first 9 integers on the Mohs scale, but the interval between 9 (corundum) and 10 (diamond) represents a much larger difference in indentation hardness than a single unit on the Mohs scale would suggest.

Typical values of very hard materials are:

SiC (carborundum), Al2O3 : 9
WC, W2C, VC : 9-9.5
Diamond : 10

Indentation hardness (Knoop Scale):

Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests.

The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.

This hardness in diamond is important in its attractiveness as a gem and in its cutting efficiency as an industrial mineral. Resistance to abrasion is important as a scratch will impair the transparency, lustre and brilliancy of the stone. The hardness enables diamond to be highly polished to achieve its adamantine lustre and the facet edges to be sharp and remain so.

Note:

There are many other scales, some appropriate to all materials, some only to metals. A Knoop indenter produces a wedge-shaped indentation in the form of a parallelogram with one diagonal at least seven times longer than the other diagonal, and this method is generally considered to be the most accurate for crystalline solids. On any scale diamond is the hardest known material.

Other Properties of Diamond.